Oh....I've done this at times.

Usually it's because I can never think of something to say to add to the conversation. Saying "thanks for reading" or "Agreed" or "great insight" doesn't add a whole lot to the conversation and is almost a waste of time for the writer to click back through to see that.

You're right, though. It needs to go both ways. Thanks for the reminder that there's comments I've been meaning to go back to and have yet to do so. (I don't mean that sarcastically, I have comments pinned to go back to.)